This invention is another improvement to the pulp-wood grinding machine described in U.S. Pat. No. 4,669,166, issued Jun. 2, 1987, to the present Applicant, David B. Grimes. Both that invention and this one have been assigned to Montague Industries, Inc. (d.b.a. Montague Machine Co.) of Turners Falls, Mass.
The present invention relates to lathes and more particularly to lathes that sharpen the cylindrical grinding stones used in pulpmaking machinery.
As described in U.S. Pat. No. 4,669,166, since the invention of "pulp" paper, wood has been traditionally ground into pulp. This soft moist mass is then combined with various liquids and additional fibers to produce the type of paper most often used today.
To make the pulp, large grinders have been used. One such machine is the Great Northern Waterous Grinder manufactured by Montague Industries, Inc.
To use the "Great Northern" (shown in FIG. 1), logs are first loaded into a top chute or hopper. From there, they fall into an underlying pocket or grinding chamber where they are pressed against a cylindrical grinding stone by a hydraulically operated piston. Rotations of the stone break down the logs and grind them into fibers. The fibers are mixed with water to form the pulp.
The "pulp stone" has a series of helical grooves in its grinding surface. These grooves spiral around the stone's central, rotational axis parallel to one another.
During the stone's rotation, these grooves help to break down the logs. The high-speed abrasion, caused by shoulders of the grooves contacting the logs, breaks down lignin in the wood allowing wood fibers to separate from the log.
Over time, the outer surface of the cylindrical stone becomes worn. The side shoulders or edges of the grooves become dull and the grooves become shallow. At that point the grooves need to be resharpened or "dressed".
As with most grinders, the Great Northern Waterous Grinder is equipped with its own lathe. This lathe has a dressing wheel, or burr, which is mounted on a hydraulically or electrically activated carriage to travel axially along the outer surface of the grinding stone. As it moves, the burr resharpens the dull grooves. Ideally, the grooves should attain their original depth, and cutting shoulders of the grooves should attain their original sharpness.
In a typical pulp grinding operation, the grinding machine runs continuously from day-to-day. The diameter of the grinding stone is approximately sixty-seven inches (67"), and it typically rotates at between two hundred and fifty (250) and three hundred and fifty (350) revolutions per minute. The diameter of the burr is approximately five inches (5"). The burr has no independent rotational power and, upon contact with the grinding stone, rotates in direct proportion to the rate of rotation of the stone.
The helical grooves on the grinding stone need to be dressed approximately once per day. It is impractical to stop or slow down the stone to dress the grooves. The precision of regular dressing, or resharpening, operations will largely determine the duration of the useful life of the grinding stone. Grinding stones may last only six (6) months or as long as three (3) years.
A critical step in the dressing operation is setting the burr at the proper depth so that it does not take off too much of the surface of the grinding stone. The stone is typically a matrix of very hard abrasive particles in a porous glass medium. The hard surface of the burr has the ability to rapidly crush too much of the stone's surface. Additionally, the surface of the burr has helical ridges that are to ride in the aligned grooves on the surface of the grinding stone. If the burr is set too deeply, not only will too much of the stone's surface be removed, but also the ridges may not be properly aligned with the grooves, causing unnecessary distortion of the grooves.
Currently, most lathes set the depth of the burr by having an operator physically observe a "spark point". The burr is mounted on a bottom end of a plunger, which is manually lowered and raised by a hand wheel or crank attached to the plunger by a screw assembly. A calibrated disk on the screw assembly allows the operator to measure the position of the plunger and burr.
As the plunger descends, the operator awaits visual observation of sparks indicating contact between the burr and grinding stone. The operator then manually sets a plunger stop based upon the position reading from the calibrated disk. Once contact between burr and stone is made, the plunger is set to descend a specific, minimum depth beyond the contact point and the lathe then moves the burr axially along the surface of the grinding stone. One pass along a four-foot stone is typically adequate and usually takes between two (2) and three (3) seconds.
When the grinding stone is to be dressed on the next day, the operator starts a search for the "spark point" based upon the previous day's position readings from the calibrated disk. The readings are posted in the machine's log book.
Some lathes measure the necessary depth of plunger descent by affixing a sensor to the burr. The burr rotates when it contacts the surface of the grinding stone and the sensor detects the rotation and signals the operator.
One of the problems of the "spark point" burr-setting procedure is that the actual depth of the plunger descent is dependent upon human reaction time. However accurate a specific operator may be, subsequent operators will not react at the same speed. Consequently, inconsistent depth settings are invariably produced and the grinding stone simply cannot achieve its maximum life span.
Another problem is that the "spark point" system is readily susceptible to substantial waste as a result of human error. An operator may improperly write down a specific measurement entry in the log of the grinding machines, or misread the entry. The result could easily lead to the plunger descending rapidly too far, forcing the burr to destroy a segment of the stone. Moreover, an operator may simply neglect to set the plunger stop and allow it to descend too far.
A significant problem with the "rotation sensing" burr-setting mechanism is that any sensor on or near the burr is exposed to an extremely harsh environment. The burr is routinely exposed to intense vibration, condensing steam, corrosive liquids, high pressure hoses and rapid temperature fluctuations. A sensor affixed to or near the burr would involve considerable maintenance and cost problems.
Accordingly, it is the primary object of the present invention to provide an improved apparatus which electronically stops the movement of a lathe plunger immediately upon contact between a burr affixed to the plunger and a surface to be sharpened by the lathe.
It is another object to provide an apparatus which electronically measures and records the position of a lathe plunger so that each time a sharpening cycle is commenced the plunger can rapidly move to the previously recorded position instead of slowly searching for a contact point between the burr affixed to the plunger and the surface to be sharpened by the lathe.
It is yet another object to provide an apparatus, commensurate with the above-listed objects, where its components are not exposed to harsh environmental conditions adjacent to the surface being sharpened.
It is a still further object to implement the present invention through a simple retrofit process on an existing pulpwood grinding machine.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.